Forward class keyword arguments in mypyc

[MarthalaJagruthiReddy]

Fixes #21938

Class-definition keyword arguments were preserved in mypy's AST but silently dropped by mypyc. This change forwards them through the native and non-native class-generation paths.

Specifically:

• forwards keywords to __init_subclass__ for native extension classes
• forwards keywords to the metaclass constructor for non-native classes
• excludes metaclass= because Python consumes it for metaclass selection
• evaluates keyword expressions before __prepare__ and the class body

Tests:

• added native and non-native runtime regression coverage
• updated IR expectations for the expanded runtime operation
• mypyc class runtime suite: 125 passed
• mypyc class IR suite: 64 passed
• mypyc self-check: passed
• pre-commit hooks: passed

I used an AI coding assistant to help investigate the issue and draft the implementation; I reviewed the resulting code and test behavior locally.

[p-sawicki]

setup_non_ext_dict calls __prepare__ of the metaclass which it looks like should also receive the keyword args.

This test works with interpreted python and fails when compiled:

from typing import Any, MutableMapping
from mypy_extensions import mypyc_attr

seen: dict[str, object] = {}

@mypyc_attr(native_class=False)
class Meta(type):
    @classmethod
    def __prepare__(
        mcls, name: str, bases: tuple[type, ...], /, **kwargs: Any
    ) -> MutableMapping[str, object]:
        global seen
        seen = kwargs
        return {}

@mypyc_attr(native_class=False)
class Base(metaclass=Meta):
    def __init_subclass__(cls, **kwargs: object) -> None:
        pass

@mypyc_attr(native_class=False)
class Child(Base, marker=1):
    pass

def test_prepare_received_class_keywords() -> None:
    assert seen == {"marker": 1}, seen

test_prepare_received_class_keywords()